This invention relates to fast-drying gypsum products. More specifically, it relates to a gypsum slurry and a wallboard that require less drying time or less energy than conventional products.
Gypsum-based building products are commonly used in construction. Wallboard made of gypsum is fire retardant and can be used in the construction of walls of almost any shape. It is used primarily as an interior wall and ceiling product. Gypsum has sound-deadening properties. It is relatively easily patched or replaced if it becomes damaged. There are a variety of decorative finishes that can be applied to the wallboard, including paint and wallpaper. Even with all of these advantages, it is still a relatively inexpensive building material.
One reason for the low cost of wallboard panels is that they are manufactured by a process that is fast and efficient. A slurry, including calcium sulfate hemihydrate and water, is used to form the core, and is continuously deposited on a paper cover sheet moving beneath a mixer. A second paper cover sheet is applied thereover and the resultant assembly is formed into the shape of a panel. Calcium sulfate hemihydrate reacts with a sufficient amount of the water to convert the hemihydrate into a matrix of interlocking calcium sulfate dihydrate crystals, causing it to set and to become firm. The continuous strip thus formed is conveyed on a belt until the calcined gypsum is set, and the strip is thereafter cut to form boards of desired length, which boards are conveyed through a drying kiln to remove excess moisture. Since each of these steps takes only minutes, small changes in any of the process steps can lead to gross inefficiencies in the manufacturing process.
The amount of water added to form the slurry is in excess of that needed to complete the hydration reaction. Some of the water that is added to the gypsum slurry is used to hydrate the calcined gypsum, also known as calcium sulfate hemihydrate, to form an interlocking matrix of calcium sulfate dihydrate crystals. Excess water gives the slurry sufficient fluidity to flow out of the mixer and onto the facing material to be shaped to an appropriate width and thickness. While the product is wet, it is very heavy to move and relatively fragile. The excess water is removed from the board by evaporation. If the excess water were allowed to evaporate at room temperature, it would take a great deal of space to stack and store wallboard while it was allowed to air dry or to have a conveyor long enough to provide adequate drying time. Until the board is set and relatively dry, it is somewhat fragile, so it must be protected from being crushed or damaged.
To dry the boards in a relatively short period of time, the wallboard product is usually dried by evaporating the extra water at elevated temperatures, for example, in an oven or kiln. It is relatively expensive to build and operate the kiln at elevated temperatures, particularly when the cost of fossil fuels rises. A reduction in production costs could be realized by reducing the amount of excess water present in set gypsum boards that is later removed by evaporation.
Another reason to decrease water is that the strength of gypsum products is inversely proportional to the amount of water used in its manufacture, especially in full density slurries. As the excess water evaporates, it leaves voids in the matrix once occupied by the water. Where large amounts of water were used to fluidize the gypsum slurry, more and larger voids remain in the product when it is completely dry. These voids decrease the product density and strength in the finished product.
Dispersants are known for use with gypsum that help fluidize the mixture of water and calcium sulfate hemihydrate so that less water is needed to make a flowable slurry. Naphthalene sulfonate dispersants are well known, but have limited efficacy. Polycarboxylate dispersants are commonly used with cements and, to a lesser degree, with gypsum. The class of compounds represented by the term “polycarboxylate dispersants” is huge, and it is very difficult to predict how individual compounds react in different media.
Despite the large amount of prior art to polycarboxylate dispersants, it is difficult to predict the effect of any particular compound on the products with which it is used. Polycarboxylates are generally known to improve fluidity in cement. This does not necessarily mean that every polycarboxylate will produce the same result in gypsum products. Gypsum and cement form different crystal patterns that may disperse differently in a polycarboxylate solution. The set times of these hydraulic materials is very different, making the retardive effects of some polycarboxylates that are negligible in cement critical to the set of a gypsum wallboard. There are even variations within the realm of gypsum products, with some polycarboxylates being effective for certain gypsum sources and not others. The lack of predictability of polycarboxylate efficacy in gypsum or cement makes it difficult to make a low-water wallboard product given the constraints of the manufacturing process.
Another disadvantage is that polycarboxylates are known to interact with other additives in gypsum products. For example, foam may be added to gypsum board to reduce the weight of the board. However, some polycarboxylates destabilize some foams, causing it to collapse and lose its effectiveness before the board sets. Reaction of the polycarboxylate with foam is not generally predictable from knowledge of the chemical structure of the particular polycarboxylate dispersant being used.
In addition to acting as a dispersant, polycarboxylates are known to retard the set of hydraulic slurries. Retardation in the set of a cement slurry by several minutes would be negligible. However, on a high-speed wallboard line, set retardation of minutes could result in board too soft to cut, too fragile to move to the kiln or even too soft to convey on the production line. Wallboard should be approximately 50% set when it is cut at the knife in order to withstand subsequent handling. When high dosages of polycarboxylates are used to make a flowable slurry at low water to stucco ratios, the set time can be delayed enough to require reduction in the speed of the board making line, severely reducing efficiency. Thus, retardation of the product set times of only minutes has the potential to reduce the productivity of a board line by half, while such a delay in the processing of cementitious materials would not be noticed.
Further, retardation of the set times by the use of polycarboxylate dispersants cannot always be overcome by the addition of set accelerators. The addition of set accelerators to the mixer decreases the set time, but also causes the formation of dihydrate crystals before the slurry leaves the mixer, resulting in premature thickening and reduced flowability of the slurry. Thus, the use of set accelerators to overcome the set retardation can defeat the purpose of adding the polycarboxylate dispersant to increase flowability in the first place.
It would be a great improvement if a gypsum slurry were developed from which wallboard could be made that did not require kiln drying or long drying times. Moreover, the improved slurry would dry quickly without the increase in set times associated with the use of polycarboxylate dispersants.